The invention relates to an electronic conductor comprising a base conductor made of copper or containing mainly copper and provided with a coating of tin-lead or of tin.
Such electronic conductors are known and are used on a large scale in electronic equipment, for example on printed circuit boards, or as terminals of electronic components.
The coating of tin-lead or of tin protects the base conductor, so that corrosion of the base conductor is prevented and furthermore the coating facilitates the making of connections between various electronic components by soldering.
A problem with such electronic conductors is that material from the coating, in particular tin and copper of the base conduit tend to interdiffuse and react with each other, so that the intermetallic compounds Cu.sub.3 Sn and Cu.sub.6 Sn.sub.5 are formed which are more difficult to solder. Said intermetallic compounds can be soldered only by the process of wetting which has the disadvantage that in soldering by modern automatised techniques the time may not be sufficiently long to promote wetting and hence good electrical contact.
In particular if the coating is very thin, for example about 5 .mu.m, it is possible that the tin of the coating will react completely with the copper of the main conductor, so that only Cu.sub.3 Sn is formed and soldering is hardly possible. This will occur, for example, as a result of storage of the electronic conductor during a relatively short period of time, for example about six weeks.
In order to deal with this problem, it has been proposed to arrange a diffusion barrier, comprising for example iron, or nickel or cobalt, between the coating and the base conductor, said diffusion barrier reducing the diffusion of tin from the coating into the base conductor and consequently the formation of intermetallic compounds. Such a diffusion barrier is described in U.S. Pat. No. 3,872,356 which was published on Mar. 18, 1975.
It is an object of the invention to provide an electronic conductor of the above kind having an improved diffusion barrier reducing the transfer of tin from the coating into the base conduit to a greater extent than the known diffusion barriers.
It is a further object of the invention to allow the application of a thinner coating of tin-lead or of tin, in the presence of the diffusion barrier, and thereby to effect a more economic use of the coating metal.